Electroless copper plating per se is well known in the prior art. An electroless or autocatalytic copper plating bath usually contains a cupric salt, a reducing agent for the cupric salt, a chelating or complexing agent and a pH adjuster. In addition, if the surface being plated is not already catalytic for the deposition of the copper, a suitable catalyst is deposited on the surface prior to contact with the copper plating bath. Among the more widely employed procedures for catalyzing a substrate is the use of a stannous chloride sensitizing solution and a palladium chloride activator to form a layer of metallic palladium particles.
In addition, during the plating, the bath is generally agitated to maintain the stability of the bath and to assist in dislodging hydrogen bubbles that tend to form on the surface of the substrate being plated. One means to provide agitation is by bubbling an oxygen containing gas such as air as exemplified by U.S. Pat. No. 2,938,805 to Agens through the bath.
Although the technology relative to electroless copper plating is continually being improved whereby copper coatings having the ability to withstand severe mechanical testing without electrical or mechanical failure by cracking as exemplified by U.S. Pat. No. 3,844,799 to Underkofler and Zucconi has occurred over the last few years, certain problems still exist. Certain problems are particularly pronounced when preparing articles of very high quality such as those to be employed in printed circuit applications, e.g., printed circuit boards which contain high density circuitry and large numbers of holes such as through holes and blind holes. The problems encountered include the formation of voids in the coatings located in the holes. This in turn can cause unreliable electrical connections in the holes. In addition, even if the electrical connections initially are adequate, the presence of voids tend to cause the coatings within the holes to crack during use of the circuits. During operation, integrated circuit boards tend to expand and contract somewhat. Any discontinuities in the coating represent a prime site for cracking due to the mechanical stress from such expansion and contraction.
In addition, a major reason for yield loss in electroless plating is the formation of what is known as extranious copper or nodules. The formation of nodules in unwanted areas on a substrate can result in short circuiting by forming contact between circuit lines on the substrate. In addition, such processes as providing protective coatings providing solder and pin insertion are adversely affected by the presence of nodules on the surface.
Accordingly, the present invention provides an improved electroless copper plating process which significantly reduces, if not substantially eliminates, the presence of voids within the holes of a substrate. In addition, the present invention significantly reduces, if not entirely eliminates, the formation of nodules on the surface of the substrate.
In manufacturing printed circuits, usually an initial electroless plating operation is employed which is generally referred to as a strike bath followed by a subsequent electroless plating employing the main bath or followed by a subsequent electrodeposition plating process in order to obtain the desired thickness of the copper layer. The strike bath is formulated in order to promote the initial copper plating on the catalytic surfaces. Generally, the substrates are subjected to a strike bath for about 1 hour and then transferred to the main additive electroless copper plating bath for an additional 15 to 20 hours. The strike bath is formulated by design to be much more chemically active than the main additive bath. The use of a strike bath is disadvantageous from an economical standpoint since it requires the preparation of two separate baths for the coating process. Also, since the strike bath is more chemically reactive than the main bath, it is more difficult to control thereby contributing to the yield loss of the coated substrates.
The present invention makes it possible to eliminate the strike bath. The process of the present invention makes it possible to use only one bath which both initiates the plating and is used as the main plating bath in a continuous plating operation. The above advantages are achieved without sacrificing the stability of the plating bath.
It has been found according to the present invention that by careful control of the dissolved oxygen level in the plating bath during plating, that the above objectives and advantages are achieved. As mentioned hereinabove, the use of oxygen in plating baths has previously been suggested. However, it has not heretofore been recognized that a control of a particular range of dissolved oxygen in the plating bath would result in the advantages achieved by the present invention. In fact, the discussions in the prior art concerning the use of oxygen actually lead away from the present invention.
For instance, U.S. Pat. No. 2,938,805 to Agens suggests aerating a copper electroless plating bath by employing an oxygen-containing gas which can be diluted with any inert gas such as nitrogen. This patent does not suggest employing the particular amounts of dissolved oxygen in the bath which are necessary to achieve the results of the present invention. In fact, this patent suggests that no advantage would be obtained in using a gas containing less than the about 20% oxygen in air (along these lines, see column 3, lines 63-67).
U.S. Pat. No. 3,900,599 to Feldstein suggests removing as much as possible of oxygen from a plating bath prior to the plating operation. In addition, Example 2 of said patent clearly indicates that air agitation should not be carried out in order to avoid plating skips. It is noted that the electroless plating baths actually employed by Feldstein are flash or strike electroless baths since subsequent electrodeposition is contemplated (see column 1, lines 65 et seq. of Feldstein). Furthermore, the plating baths explicitly suggested in Feldstein are not affected by the presence of nitrogen whereas the use of nitrogen alone in the main plating bath can adversely affect its stability. Accordingly, Feldstein, as well as U.S. Pat. Nos. 3,666,527 to Feldstein and 3,454,416 to Heymann et al, actually suggest that oxygen should not be deliberately or purposely introduced during the electroless plating of copper.
Horvath et al in U.S. Pat. No. 3,300,335 suggest foaming an electroless plating bath prior to contacting it with the substrate to be plated by employing a gas which can be air or an inert gas such as nitrogen. Again, Horvath et al fail to suggest maintaining the oxygen level at the values required by the present invention in order to achieve the results obtained herein.
Discussions of the affect of oxygen with respect to nickel plating can be found, for instance, in Chemical Abstracts, Volume 76, page 62385g and Chemical Abstracts, Volume 72, page 92799e. In addition, the use of certain light inert gases such as hydrogen, helium, methane, or neon in electroless nickel plating is discussed in U.S. Pat. No. 2,819,188 to Metheny et al.